External filter cakes are used to minimize fluid loss and solids invasion to a formation from drilling and completion fluids. Subsequently, the cake must be removed to increase the flow area and minimize skins, especially for openhole and gravel-packed completions in horizontal and deviated wells. Experimental data is presented to show that the pressure required to initiate flow into the wellbore after building up a filter cake is affected by rock permeability; mud properties, such as particle size and cake yield strength; flowback velocity; and overbalance pressure.

Mudcake liftoff tests, mud particle size, and rock pore-throat size distribution measurements were performed to understand the factors that contribute to cake removal. The mechanisms that control filter-cake removal are discussed.

In this study, it is clearly shown that the flow-initiation pressure during flowback is controlled by solids and filtrate invasion (i.e., internal formation damage rather than by the external mudcake). Flow-initiation pressures show a minimum with increasing rock permeability caused by two competing effects. Larger pore sizes result in smaller flow-initiation pressures, ?P_fi; however, more solids invasion increases ?P_fi. Higher overbalance pressures also increase the internal formation damage and flow-initiation pressure. A simple model to calculate the flow-initiation pressure during flowback is proposed. The model correctly predicts the experimentally observed trends with rock permeability, mud particle size distribution, extent of solids invasion, and yield strength of the mudcake. The model provides a systematic method for designing fluids with low flow-initiation pressures. It is also shown in this study that low flow-initiation pressures do not imply complete cake removal. Cake removal is primarily controlled by the permeability of the cake and its mechanical properties.